Commonly-owned U.S. Pat. Nos. 5,475,355 and 5,495,222, whose contents are herein incorporated by reference, describe, among other things, a generally C-shaped magnetic structure for generating uniform magnetic fields in a region of interest, for example, for use in magnetic resonance imaging (MRI) by means of a relatively open magnetic structure allowing surgical intervention during an MRI procedure. Among the features described in those two patents are various schemes for compensating for field distortions introduced by, among other things, the opening of the structure.
A commonly-owned, copending application, Ser. No. 08/754,916, filed Nov. 22, 1996, whose contents, including the contents of the patents incorporated by reference in the said copending application and the contents of a continuation-in-part application, Ser. No. 08/885,095, are herein incorporated by reference, describes apparatus for generating uniform magnetic fields in a region of interest employing wedge-shaped blocks as the primary generator of the main magnetic field. A major thrust of this application is how to generate a field that is sufficiently uniform for use in MRI by means of a relatively open magnetic structure allowing surgical intervention during an MRI procedure.
While the magnetic structures described in these referenced patents and applications are suitable for their intended purpose, nevertheless a possible shortcoming is the generation of a substantial fringe field in the medium surrounding the magnetic structure as a result of opening up the structure for surgical intervention. The fringe field can undesirably interfere with a surgeon""s use within or adjacent the magnetic structure of various instruments and equipment which may be of ferromagnetic material and may be subject to magnetic field forces.
The parent application, Ser. No. 09/069,389, whose contents, including the contents of the patents and publications incorporated by reference in the said copending application, are herein incorporated by reference, describes a new open magnetic structure for MRI with two principal novel features. The first is what is referred to as a unipolar structure. Most permanent magnetic structures employed in this field possess two distinct opposed poles, and the main magnetic field through the region of interest in a cavity in the magnetic structure flows directly between the opposed pole faces. The unipolar structure of the parent application is unusual because it has only one distinct pole which faces the opening provided for accessing the region of interest, the opening normally being the place where the second pole would be located if the opening were not present. The main magnetic field still flows from the unipole through the region of interest but is now collected by the magnetic members adjacent the opening. As explained in that application, opening such a structure to provide access to the cavity provides field perturbations that are unacceptable for MRI applications. The second novel feature is to configure the structure to form at the location of the opening, i.e., where the cavity interfaces with the non-magnetic external medium, an equipotential surface. This is the equivalent of the equipotential surface formed by having a high-permeability membrane parallel to the single pole piece present which closes off the interface and thus the opening. This design constraint was described as a hypothetical high permeability membrane present where the open second side of the cavity will be made, the high permeability membrane forming the desired equipotential surface at the interface with the surrounding medium. The presence of the equipotential surface at the opening represented by the hypothetical high permeability membrane, in effect, extends the uniform field up to the opening. This minimizes the fringe field generated in the surrounding medium when the second side of the cavity, when this hypothetical high permeability membrane is removed, is opened.
A limitation of that open unipolar magnetic structure is that the region of interest is most uniform close to the bottom side or first side of the cavity which comprises a high permeability ferromagnetic member interfacing with the first side of the cavity and forming the single pole piece of the magnetic structure. There may be applications of an open unipolar magnetic structure where it is desirable to be able to arbitrarily locate the region of interest within the cavity. Usually, it would be preferred to move the region of interest closer to the opening at the top or second side of the cavity simplifying access by a surgeon to the part of a patient located in the region of interest.
A xe2x80x9chigh permeabilityxe2x80x9d member means a soft ferromagnetic body having a permeability xcexc exceeding 10 and preferably as high as possible. In several of the referenced patents and publications, as well as in this application, it is sometimes referred to as a (xcexc=∞) material, in other words, an ideal soft ferromagnetic body. Practically speaking, the differences in function of a (xcexc=∞) material and a high permeability material are small. The behavior of infinite permeability ferromagnetic material can be approximated to a high degree of accuracy by soft iron as long as it is not magnetically saturated.
A xe2x80x9cyoked magnetic systemxe2x80x9d means a permanent magnetic structure surrounded by a body of high permeability materialxe2x80x94typically called a yokexe2x80x94that functions to carry the return flux of the induction B from one pole of the magnetic structure to the other pole. To perform this function, typically the yoke requires a significant cross sectional area sufficient to convey the flux without saturating, and often has a varying cross section in accordance with the local flux-carrying requirement.
A xe2x80x9cstrapxe2x80x9d means a high permeability member that functions to establish a unipotential or equipotential surface within or at the surface of a permanent magnet that may have inhomogenieties due to manufacturing tolerances and whose interior region or a surface thus deviates from what should have been an equipotential surface. In other words, the strap functions to create the equipotential condition that would have existed if the magnet were ideal with perfect materials and geometry. An external strap is somewhat equivalent to the yoke of a magnet, the basic difference between the yoke and the external strap being that the strap function is limited to closing the flux of the statistical fluctuation of the magnetic induction, i.e., to eliminate the components of the field parallel to the strap surface, while the yoke is designed to close the entire flux generated by the magnet. Since the strap only carries the flux of the statistical fluctuation of the magnetic induction, it requires only a small cross sectional area and typically is a very thin layer of uniform thickness. For a further description, reference is had to a copending application, Ser. No. 08/613,756, filed Feb. 26, 1996, now U.S. Pat. No. 5,798,680, and a paper entitled xe2x80x9cStrapping Techniques For Permanent Magnets, IEE Transactions On Magnetics, 32/5, September 1996, Pgs. 5082-5084, whose contents are herein incorporated by reference.
xe2x80x9cUnipolar magnetic structurexe2x80x9d is used herein in the sense that a single magnetic pole piece of a magnetic structure functions as a platform adjacent a cavity for receiving an object to be analyzed, the single magnetic pole generating flux in the region of interest within the cavity, whereas the magnetic structure, rather than a second distinct pole piece, collects that magnetic flux. In a unipolar magnetic structure, there typically will not exist a single opposed magnetic element that can perform the function of a second magnetic pole piece.
A xe2x80x9cuniform magnetic fieldxe2x80x9d means a magnetic field that over a region of interest has an intensity that varies by less than 100 ppm.
The term xe2x80x9csubstantiallyxe2x80x9d appears from time to time in the specification and claims. It should be understood in a practical sense to mean small variations in some structure or parameter term that do not make significant changes in properties associated with those structures or parameters and are included within the scope of the associated term. For example, xe2x80x9csubstantially uniformly-magnetized blocksxe2x80x9d recognizes that, while identical magnitude and orientation of the field in similarly configured blocks is desired, the usual material and dimensional tolerences do not allow this ideal to be attained. Where a xe2x80x9csubstantially plane surfacexe2x80x9d is defined, this is meant to include the addition to the surface of, for example, thin filter structures or other field distortion compensating means which could result in uneveness but in which the surface would still be essentially planar.
The principal object of the invention is a improved open unipolar magnetic structure allowing an arbitrary location of the region of interest which will contain the uniform magnetic field.
This object is achieved in accordance with one aspect of the invention by a yoked magnetic structure that is modified to reduce the perturbations in the region of interest when the latter is arbitrarily located within the cavity. Put another way, the region of uniform field is extended outside the opening with the result that the region of interest can, in a preferred embodiment, be moved closer to the opening.
In a preferred embodiment, the modification includes the provision of supplemental permanent magnetic members at the opening, the configuration of the supplemental permanent magnetic members being related to the configuration of some of the permanent magnetic members of the magnetic structure that provides the main magnetic field.
In a further preferred embodiment of the invention, two of the permanent magnetic members of the magnetic structure have a triangular or wedge shape with an apex angle determined by a design parameter K of the structure. (The design parameter K is explained in several of the referenced prior applications.) The supplemental permanent magnetic members are attached to the ends of the yoke defining the opening, and those supplemental permanent magnetic members are also triangular or wedge-shaped and have substantially the same apex angle.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described the preferred embodiments of the invention, like reference numerals or letters signifying the same or similar components.
In the drawings:
FIG. 1 is a perspective view of one form of open unipolar magnetic structure in accordance with the invention with some typical outside dimensions included;
FIG. 2 is an end or cross-sectional view of the embodiment of FIG. 1 showing additional details of the structure;
FIG. 3 is a graph illustrating for one particular design the effect on the field within the cavity as the height of the structure adjacent the opening is varied;
FIG. 4 is an enlarged cross-sectional view of the plane high-permeability member that interfaces with the cavity present in the embodiment of FIG. 1.